1. Field of the Invention
The present invention generally relates to image sensors. More particularly, the present invention relates to image sensors configured to reduce dark current and to methods of manufacturing images sensors to reduce dark current.
2. Description of the Related Art
Certain types of image sensors utilize photodiodes to capture incident light and convert the light to an electric charge capable of image processing. Examples include Charge Coupled Device (CCD) image sensors and Complimentary Metal Oxide Semiconductor (CMOS) image sensors (CIS), respectively illustrated in FIGS. 1 and 2. The CCD sensor of FIG. 1 is generally configured by an array of photo-detectors that are electrically connected to vertical CCDs functioning as analog shift registers. The vertical CCDs feed a horizontal CCD which in turn drives an output amplifier. In contrast, the CIS device of FIG. 2 is characterized by an array of photo detectors have access devices (e.g., transistors) for connection to word lines and bit lines. The word lines are connected to a row decoder circuit, and the bit lines are connected to a column decoder circuit through column amplifiers. The column amplifiers drive an output amplifier as shown. The configuration of the CIS device is analogous to that of a CMOS memory device.
One drawback with the used of photodiodes relates to their propensity to accumulate electrical charge in the absence of incident light. The result is commonly referred to as “dark current”. Dark current from a photodiode may manifest itself as a “white” pixel in the processed image, thus degrading image quality.
Dark current is generally caused by a number of different factors, including plasma damage, stresses, implant damage, wafer defects, electric fields, and so on. However, one particularly major source of dark current is dangling silicon bonds which exist on the surface of the silicon substrate of the image sensor. At relatively high thermal ranges, these dangling silicon bonds generate negative charges that can be accumulated by the photodiode even in the absence of incident light. Such high thermal ranges can occur, for example, when a cell phone having an image sensor is utilized for an extended period of time.
There is a general demand in the industry for image sensors which exhibit reduced dark current, such as the dark current caused by dangling silicon bonds on a silicon substrate surface.